Motor vehicle automatic shift transmissions are generally equipped with fluid operated torque transmitting elements (referred to as clutches or brakes) which are selectively engageable to establish various speed ratio drives between the input and output shafts of the transmission. The shift points at which the transmission is shifted from one forward speed ratio to another are empirically determined to satisfy desired performance and/or economy concerns, and are typically scheduled as a combined function of the driver demand (engine throttle opening or accelerator pedal position) and the vehicle speed.
With electronic controls, the shift points are mapped to form a shift pattern look-up table in nonvolatile memory, the shift point lines being defined as a two-dimensional function of driver demand and vehicle speed. A representative look-up table for a four-speed automatic transmission is graphically depicted in FIG. 2. The table is addressed as a function of the driver demand term and yields a previously stored vehicle speed value which must be reached (exceeded in the case of an upshift) before a shift can occur. When the vehicle speed condition is met, the desired speed ratio is changed accordingly. If the desired ratio is not engaged, the transmission controller schedules a shift to the desired ratio. Although not shown, hysteresis between upshift and downshift lines is typically employed to minimize hunting.
The sophistication of electronic controls also permits a relatively comprehensive transmission failure analysis. For example, speed ratio progression during shifting operation and clutch slippage during nonshifting operation may be monitored to detect the occurrence of a ratio failure.
In the event of a failure, the shift pattern look-up table must somehow be modified in order to avoid damaging the transmission by shifting to a failed speed ratio. In the case of a failure of the lowermost or uppermost speed ratio, the failed ratio simply becomes unavailable. In the case of a failure of an intermediate speed ratio, however, the control becomes more difficult. While a different ratio could be substituted for the failed intermediate ratio (so that a normally scheduled shift to the failed ratio would result in a shift to the next higher or next lower ratio), the resulting shift is often unacceptable since the substituted gear is generally not suitable for the vehicle speed and load condition. As a result, engine stalling or overspeeding, and stressing of the transmission gear elements may occur.